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BELL SYSTEM TECHNICAL JOURNAL 



beside a Geiger-Miiller counter.^ Hundreds of counts per minute 

 disclosed the emergence of fast-flying particles from the samples. 

 The number per minute fell off exponentially (Fig. 1) with the lapse of 

 time: a very important feature, for this is the law of radioactivity. 

 The exponential decline implies that the nuclei which were destined 

 to emit these particles were formed at the moments of collisions and 

 existed intact for periods of time — "lifetimes" — not the same for all 

 but distributed in a perfectly random fashion. Such a decline is 



2.0 

 1.9 

 1.8 



1.7 



>- 



<n 1.6 



z 



UJ 



t- 

 Z 1.5 



I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 



TIME IN MINUTES 



Fig. 1 — Exponential decay of the radioactivity induced by boron, aluminium, 

 magnesium with alpha-particles: semi-logarithmic plot. (F. Joliot & I. Curie- 

 Joliot, Journal de Physique.) 



characterized by a singe constant, the "half-period," or lapse of time 

 during which the rate of emission of particles drops to one-half of its 

 initial value. The half-periods in the three cases examined by the 

 Joliots are different: boron 14', magnesium 2' 30", aluminium 3' 15". 

 This is a welcome feature wherever it occurs, as when two substances 

 exhibit different half-periods the effect cannot be ascribed to any 

 contamination common to both.^ 



Since thus there are not only delays between the bombardment and 

 the ultimate disruption, but also (at any rate in the tested cases) a 



1 "The Nucleus, Second Part," p. 119; the "Geiger-Miiller" counter has a thin 

 wire for its inner electrode, while most of those called simply " Geiger counters" have 

 needle-points, though the earliest counters invented by Geiger were of the former 



2 Cases are on record in which several different elements have exhibited decay- 

 curves, each the sum of two exponentials, one having a half-period characteristic 

 of the element and the other a half-period common to all samples; the latter is then 

 ascribed to a common admixture. 



